The present invention relates generally to control rods for nuclear reactors having an upper, generally cruciform cross-section and a lower velocity limiter secured to the upper section and particularly relates to apparatus and methods for selectively adjusting the weight of the velocity limiter whereby the overall control rod weight can be adjusted.
Control rods in nuclear reactors perform dual functions of power distribution shaping and reactivity control. The rods enter from the bottom of the reactor and are typically connected to bottom-mounted, hydraulically-actuated drive mechanisms to allow axial positioning for reactivity regulation or rapid SCRAM insertion. The control rods are generally cruciform in cross-sectional shape, with each blade or wing of the rod typically containing tubes filled with a nuclear poison such as boron carbide or hafnium. The lower portion of the control rod comprises a velocity limiter disposed in a guide tube. Velocity limiters are conventionally designed to limit the drop velocity to a selected velocity, for example, 3.11 feet per second, minimize drag resistance during a SCRAM event, provide an interface between the control rod and control rod guide tube and provide a coupling between the control rod blade absorber sections and the control rod drive.
Historically, a single velocity limiter was developed to satisfy the foregoing design criteria. Newer control blade absorber sections, however, have been designed, each with widely differing load patterns and neutron-absorbing materials. This has resulted in some control rod absorber sections being heavier than the original design and some being lighter. Because control rod weight limits are defined by what the specific plant is licensed to use, two different types of velocity limiters became available for use with any one of the different control rod designs. Specifically, a light and a heavy velocity limiter were designed for use with any one of the control rod designs. The selection of which of the lighter or heavier velocity limiters for a particular control rod design was based on the weight of the control blade absorber section, i.e., a light velocity limiter was used with a heavier absorber section and, conversely, the heavy velocity limiter was used with a lighter absorber section. There has therefore developed a need for a single velocity limiter which enables the weight of the limiter to be modified without affecting the critical external geometric or fluid mechanical properties of the control rod such that the overall weight of the control rod can be closely controlled to meet design and licensing specifications.
In accordance with a preferred embodiment of the present invention, a velocity limiter for a control rod for a nuclear reactor is provided which can be adjusted in weight within a predetermined range such that the velocity limiter with a selectively adjusted weight can be used with any one of the various and different control rod absorber section designs. The specific weight of the velocity limiter is determined by the weight of the neutron absorber section, i.e., with a heavier absorber section, a lighter velocity limiter is provided and, conversely, with a lighter absorber section, a heavier velocity limiter is provided.
To provide a substantially infinitely adjustable weight of velocity limiter within a predetermined range of weights, the present invention provides, as part of the velocity limiter, a standard vane, a set of light or heavy transition pieces, a socket and a set of light or heavy fins. By selecting one or the other of the light or heavy transition pieces and combining the selected transition piece with one or the other of a selected one of the light or heavy sets of fins, an approximation of the desired weight for the velocity limiter and, hence, the overall control rod, can be obtained. To provide a further fine adjustment in weight to match a target or originally licensed control rod weight, the heavy transition piece can be modified such that its weight lies within a range from approximately the weight of the light transition piece to the weight of the heavy transition piece without modification. Each transition piece includes an enlarged diameter central section having a bore therethrough. The heavier transition piece has a small bore whereas the light transition piece has a radially enlarged bore. By removing material from within the bore of the heavy transition piece, the weight of the heavy transition piece can be adjusted downwardly within a weight range substantially between the weight of the light and heavy transition pieces. Consequently, by combining the heavier transition piece with the socket and one of the sets of fins, those elements can be assembled with the vane and socket to form a velocity limiter of desired weight such that the overall weight of the control rod approaches the target or licensed weight.
In a preferred embodiment according to the present invention, there is provided a method of adjusting the weight of a control rod for use in a nuclear reactor, the control rod having an upper, generally cruciform absorber section and a lower velocity limiter attached to the section and including a vane, a transition piece and fins, comprising the steps of providing first and second sets of fins with the second set of fins being heavier than the first set of fins, providing first and second transition pieces, with the second transition piece being heavier than the first transition piece, selecting one of the first and second sets of fins and one of the first and second sets of transition pieces and forming a velocity limiter for the control rod containing the vane and the selected ones of the fins and the transition pieces thereby selectively adjusting the weight of the control rod.
In a further preferred embodiment according to the present invention, here is provided apparatus for selectively adjusting the weight of a control rod for use in a nuclear reactor comprising an upper control rod body, a lower velocity limiter including a vane, a first set of fins, a second set of fins heavier than the first set of fins, a first transition piece, a second transition piece heavier than the first transition piece, the control rod being formed with one of the first and second sets of fins secured to the vane and one of the first and second transition pieces secured to the vane whereby a control rod of selected adjusted weight is formed.